Design Guidance for Freeway Mainline Ramp Terminals (2012)

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1 Design Guidance for Freeway Mainline Ramp Terminals The design of ramp-freeway junctions is a critical component in the overall safety per- formance of a controlled access facility. Interchanges present the greatest safety and opera- tional challenges for drivers on the freeway system, as most freeway crashes occur in the vicinity of interchanges. Entry and exiting maneuvers near interchanges place increased workload on drivers associated with navigational decision making, speed-changing, and tracking demands. This report focuses on the design of freeway mainline ramp terminals (i.e., acceleration and deceleration lanes). Design values for freeway mainline ramp terminals in the 2004 Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) rely on somewhat outdated research. The objective of this research is to develop improved design guidance for freeway mainline ramp terminals based on modern driver behavior and vehicle performance capabilities. This research addresses the following questions: • Is the fundamental AASHTO model or set of operational assumptions behind freeway mainline ramp terminal design sufficient to describe the full range of design parameters? • Are acceleration and deceleration rates inherent in the AASHTO speed-change lane (SCL) models appropriate for today’s driver population and vehicle fleet? • Should trucks be used as the design vehicle for freeway mainline ramp terminal design? • Are there differences in the operational performance between parallel and tapered SCLs? • Does driver behavior differ for low-speed ramps compared with higher speed ramps? • What are the implications of any design changes on current roadway practice and existing designs? The primary steps to address these key questions included a crash analysis and field stud- ies. The crash analysis was conducted to investigate the vehicle types involved in crashes near freeway mainline ramp terminals. If trucks were disproportionately represented in crashes near freeway mainline ramp terminals, an argument could be made that a truck, rather than a passenger car, should be the design vehicle for an entrance or exit ramp. Field studies were conducted to examine vehicle performance and driver behavior near freeway mainline ramp terminals. An observational field study was performed to collect speed, acceleration/ deceleration, and distance information on a large number of vehicles at several freeway entrance and exit ramps. A driver behavior study was also performed in which sub- jects drove an instrumented vehicle along a selected course using designated entrance and exit ramps. In addition to behavioral data, speed, acceleration/deceleration, and location information were also gathered during the behavioral study. Data from both studies were analyzed to address the central questions related to this research. S u m m a r y

2Several key findings from the crash analysis are as follows: • In general, there was no difference between truck crash rates and crash rates for all vehicles at either entrance or exit ramps. The exit ramp configurations where truck crash rates exceeded the overall crash rates by the greatest amount included parclo loop, free-flow loop, and “other” ramps. • Trucks are involved in approximately the same proportion of crashes in the vicinity of freeway mainline ramp terminals as they are along the mainline of urban freeways (about 10 percent). • Where the ramp truck average daily traffic (RTADT) exceeds 1,000 trucks/day, it appears that truck crashes occur more frequently compared to terminals with lower RTADT. Several key findings from the observational and behavioral studies related to entrance ramp terminals (i.e., acceleration lanes) are as follows: • Vehicles tend to merge later under constrained-merge conditions than under free- or forced-merge conditions. • The distribution of merge locations for trucks is very similar to that for passenger cars. • Vehicles merge later at tapered SCLs than at parallel SCLs. • Vehicles merge at speeds closer to freeway speeds under constrained- and forced-merge conditions than under free-merge conditions. • Late mergers enter the mainline freeway lanes at speeds closer to freeway speeds than early mergers. • The speed differential between merging vehicles and mainline freeway vehicles is nearly the same for both straight and loop ramps. • Vehicles merge closer to freeway speeds at tapered acceleration lanes than at parallel accel- eration lanes. • The median acceleration rates for free-flow passenger cars under free-merge conditions are greater than the assumed acceleration rates in the 2004 Green Book. • Entrance-ramp vehicles depart the controlling features of entrance ramps at speeds higher than assumed in the 2004 Green Book. • Acceleration rates for free-flow trucks are lower than for free-flow passenger cars. • In uncongested or lightly congested conditions, drivers tend to glance into their mirror or over their shoulder about three times before merging onto the freeway. A typical glance lasts about 2.5 to 3.0 s, and drivers increase speeds by approximately 2.5 mi/h during each glance. Several key findings from the observational and behavioral studies related to exit ramp terminals (i.e., deceleration lanes) are as follows: • Under free-diverge conditions the distribution of diverge locations for trucks is very simi- lar to that for passenger cars. • At the point where vehicles maneuver from the freeway to the deceleration lane, speeds of diverging vehicles are typically 4 to 7 mi/h below average freeway speeds. • Diverge speeds of trucks are lower than for passenger cars. • Diverging vehicles decelerate at rates lower than those assumed in the 2004 Green Book. • There is little difference between deceleration rates of passenger cars and trucks. • Deceleration rates of exiting vehicles are greater for vehicles that diverge closer to the painted nose than for vehicles that diverge further upstream from the painted nose. • Deceleration rates are greater on loop ramps than straight ramps.

3 • Deceleration rates are substantially higher on parallel SCLs than on tapered SCLs. • The conceptual approach used in AASHTO policy for the design of exit ramps, which assumes that drivers decelerate in gear (i.e., coast) for 3 s, is consistent with current driver behavior, assuming the definition of coasting includes the time used to remove the driver’s foot from the throttle. A substantial portion of this deceleration time occurs within the mainline freeway lanes. Several general conclusions related to the design of freeway mainline ramp terminals based upon the findings of this research are as follows: • Passenger cars should remain the principal design vehicle for freeway mainline ramp terminals, except where truck volumes on ramps are substantial, in which case further consideration should be given to more fully accommodating trucks within the design. • Freeway mainline ramp terminals should be designed based upon free-flow conditions. Several conclusions specific to entrance ramps based upon the findings of this research are as follows: • In free-merge conditions, many vehicles choose to enter the freeway at speeds much lower than the speed of freeway traffic. Drivers simply choose not to use the full length of the ramp and SCL when gaps are abundant and merging is not difficult. • Heavy vehicles do not perform as well as passenger cars at entrance ramps. Their accelera- tion rates are lower, and they merge onto the freeway at lower speeds. However, at ramps with a small proportion of truck traffic, their merging behavior does not appear to nega- tively impact the overall operation of the ramp terminals. • Vehicles are more likely to use the full length of a tapered SCL to accelerate to near freeway speeds before merging, in contrast to parallel SCLs where vehicles may merge earlier along the ramp and at lower speeds. • There is no substantive difference in operational performance between low-speed (loop) and high-speed (straight) entrance ramps under free-merge conditions. • The conceptual approach used in AASHTO policy for the design of entrance ramps, which assumes constant acceleration, is a reasonable approach for determining minimum acceleration lane lengths for design. In addition, the current values provided in Green Book Exhibit 10-70 are conservative estimates for minimum acceleration lane lengths, given the current vehicle fleet and driver population, and do not need to be modified. In particular, they provide sufficient length for vehicles to merge onto the freeway under a range of freeway operating conditions. In the design of entrance ramps where free-merge conditions are expected for the foreseeable future and constraints make it difficult to provide the recommended minimum acceleration lengths, the minimum acceleration lane lengths presented in the Green Book can be reduced by 15 percent without creating operational problems. Several conclusions specific to exit ramps based upon the findings of this research are as follows: • Most diverge maneuvers begin before or within the taper area or within the first or middle thirds of the SCL. Few diverge maneuvers take place in the final third of the SCL or beyond the painted nose. • Vehicles that diverge earlier along the deceleration lane diverge closer to freeway speeds than vehicles that diverge later along the deceleration lane (i.e., closer to the painted nose).

4• Where the deceleration lane length is longer than the recommended minimum length presented in the Green Book, most vehicles decelerate at rates lower than those assumed by the Green Book. This is in varying degrees due to the additional length provided and to vehicles decelerating in the freeway lane prior to initiating the diverge maneuver. • Deceleration rates of exiting vehicles are greater for vehicles that diverge closer to the painted nose than for vehicles that diverge further upstream from the painted nose. • When exiting the freeway, trucks decelerate at rates very comparable to those of pas- senger cars. In addition, trucks typically diverge from the freeway at lower speeds than passenger cars. • Crash rates for trucks are higher at parclo, free-flow, and “other” ramp configurations than at diamond; outer connection; direct or semi-direct connection; and button hook, scissor, and slip ramp configurations. • Drivers exiting on loop ramps tend to reduce their speed in the freeway lane more, and decelerate along the SCL at a greater rate, than drivers exiting on straight ramps. This may be due the visual perceptions of drivers as they approach the horizontal curvature of a loop ramp. • The geometry of parallel deceleration lanes generally leads to substantially higher decel- eration rates than on tapered deceleration lanes. This may be the result of vehicles diverg- ing slightly closer to freeway speeds along parallel deceleration lanes than along tapered deceleration lanes. The disparity between deceleration rates is most apparent on straight ramps. • The conceptual approach used in AASHTO policy for the design of exit ramps, which assumes a two-step process of deceleration, is a reasonable approach for determining minimum deceleration lane lengths for design. In addition, the current values provided in Green Book Exhibit 10-73 are conservative estimates for minimum deceleration lane lengths, given the current vehicle fleet and driver population, and do not need to be modi- fied. No critical or unusual diverge maneuvers were observed at the study sites that met and exceeded the current design criteria. In addition, at these study locations, vehicles deceler- ated at rates well within the capabilities of the vehicle fleet and driver preferences. This was in part due to some deceleration by diverging vehicles in the freeway mainline prior to the diverge maneuver. Given this last point, it is beneficial to have a conservative design process that does not assume that vehicles begin decelerating in the freeway mainline. Several potential changes for consideration in the next edition of the Green Book are pro- posed based on the findings and conclusions of this research as follows: • Include a statement in the Green Book text that tapered SCLs are preferred over parallel SCLs at entrance ramps because vehicles tend to merge closer to freeway speeds at tapered SCLs, and, if parallel SCLs are used, they are most appropriate at ramps expected to expe- rience constrained- or forced-merge conditions because they provide greater flexibility in selecting a merge location along the SCL. • Include a statement in the text accompanying Green Book Exhibit 10-70 that design val- ues in the exhibit are conservative, and where free-merge conditions are expected for the foreseeable future and where constraints make it difficult to provide the recommended minimum acceleration lengths presented in the Green Book, minimum acceleration lane lengths may be reduced by 15 percent. • Include additional exhibits that provide speed-distance curves for trucks in a range from 140 to 200 lb/hp. This will provide the designer with more flexibility to select an appropri- ate heavy vehicle for design, and in some cases find a better compromise between design- ing for passenger cars and trucks, especially at entrance terminals with substantial truck

5 volumes. These exhibits will ideally be provided in Chapter 3 of the Green Book because they could be used for more general design purposes, rather than just for acceleration lanes. A reference to the new exhibits will ideally be included in Chapter 10 of the Green Book. • Emphasize in the Green Book text that values presented for minimum deceleration lane length on exit ramps are conservative and do not account for deceleration in the mainline freeway lanes. While some drivers do accomplish a considerable portion of their decel- eration on the freeway, it is prudent for the designer not to assume deceleration in the mainline freeway lanes in the design of an exit ramp. • Mention within the text that providing deceleration lanes longer than the minimum values in Green Book Exhibit 10-73 may promote casual deceleration by exiting drivers, particularly under uncongested or lightly congested conditions. This is not necessarily a negative result, but it does change the operational characteristics of the ramp.